A common commercial requirement is to dry a compressed gas, such as air, and in particular to lower the relative humidity of the gas so that, at a designated temperature, no moisture is condensed from the gas. It is so often desirable to lower the relative humidity of compressed air, for example, from an air compressor, so that there will be no water condensed, if the temperature is reduced to below 40.degree. F; that is, the pressure dew point of the gas should be 40.degree. F or lower.
The selection of a compressed air dryer and the common types of compressed air dryers available are described, for example, in Plant Engineering of Dec. 12, 1974, pp. 74-78, herein incorporated by reference. In general, three types of compressed air dryers are commonly used: deliquescent dryers, regenerative dessicant dryers (including heatless dryers); and refrigerated dryers. Although such dryers have various advantages, there is a need for a simple, low-maintenance, maximum-efficient, low-cost, self-controlled air dryer capable of operation at remote or hazardous locations by compressed air.